1. Field of the Invention
The present invention relates to a radio transmission apparatus and a radio transmission method, and more particularly, to a radio transmission apparatus and a radio transmission method that use a quadrature modulation scheme or the like.
2. Description of the Background Art
In a mobile terminal such as a mobile phone, when there is a conductor, such as a metal or a human body, near an antenna, the characteristic of the antenna is disturbed. Thus, the output impedance of an amplifier of the mobile terminal is shifted from a desired value, and a reflected wave of a transmission signal outputted by the amplifier returns from the antenna to the amplifier. As a result, the input/output characteristic of the amplifier varies. Thus, the mobile terminal does not meet standards concerning distortion such as ACP (Adjacent Channel leakage Power). This results in a cause of communication failure such as outputting of a wave over a channel other than a specified communication channel.
In order to prevent such a problem, conventionally, an isolator is inserted between the amplifier and the antenna such that the reflected wave, which returns from the antenna because the output impedance of the amplifier is shifted from the desired value, does not reach the amplifier, thereby preventing the variation of the input/output characteristic of the amplifier.
FIG. 14 is a view showing an example of a conventional radio transmission apparatus 500 that modulates data using a quadrature modulation scheme and transmits the modulated data. The conventional radio transmission apparatus 500 will be described below with reference to FIG. 14. The radio transmission apparatus 500 includes a modulation section 501, a distortion correction section 502, a look-up table holding section (hereinafter, referred to as an LUT holding section) 503, a distortion control section 504, a digital/analogue conversion section (hereinafter, referred to as a D/A) 505, a mixer 506, an amplification section 507, a power supply section 508, an isolator 509, and an antenna 510.
The modulation section 501 generates a quadrature modulated signal using an I-phase signal and a Q-phase signal that are signals for modulation.
The LUT holding section 503 holds a look-up table (hereinafter, referred to an LUT) for correcting a distortion of an output signal of the amplification section 507.
Here, the input/output characteristic of the amplification section 507 will be described prior to description of an LUT of the LUT holding section 503. FIG. 15 shows the input/output characteristic of the amplification section 507. In FIG. 15(a), the horizontal axis indicates an electric power PIN of an input signal of the amplification section 507, and the vertical axis indicates an electric power POUT of an output signal of the amplification section 507. As shown in FIG. 15(a), due to the influence of a saturation region, the electric power input/output characteristic of the amplification section 507 becomes nonlinear. In FIG. 15(b), the horizontal axis indicates the electric power PIN of the input signal of the amplification section 507, and the vertical axis indicates a shift ΔΦ between the phases of the input and output signals of the amplification section 507. As shown in FIG. 15(b), due to the influence of a saturation region, the shift ΔΦ between the phases of the input and output signals of the amplification section 507 varies and does not become constant. In other words, the input/output characteristic of the amplification section 507 is distorted in terms of electric power and phase.
FIG. 16 represents the LUT, held by the LUT holding section 503, by using two graphs for convenience of explanation. The graph of FIG. 16(a) is used for adjusting the electric power PIN of the input signal of the amplification section 507. In the graph of FIG. 16(a), the horizontal axis indicates an electric power IN of an input signal of the distortion correction section 502, and the vertical axis indicates a gain (OUT/IN) given to the electric power IN of this input signal by the distortion correction section 502. The graph of FIG. 16(b) is used for adjusting the phase of the input signal of the amplification section 507. In the graph of FIG. 16(b), the horizontal axis indicates the electric power IN of the input signal of the distortion correction section 502, and the vertical axis indicates a phase variation Δphase given to this input signal by the distortion correction section 502.
The distortion control section 504 refers to the LUT of the LUT holding section 503 and controls the distortion correction section 502, thereby causing the distortion correction section 502 to adjust the electric power and the phase of the quadrature modulated signal generated by the modulation section 501.
In accordance with the control of the distortion control section 504, the distortion correction section 502 adjusts the electric power and the phase of the quadrature modulated signal generated by the modulation section 501, and outputs the adjusted quadrature modulated signal. Specifically, the distortion correction section 502 gives the quadrature modulated signal a gain in accordance with its electric power IN (see FIG. 16(a)), and also gives the quadrature modulated signal a phase variation Δphase in accordance with its electric power IN (see FIG. 16(b)), to generate the adjusted quadrature modulated signal.
The D/A 505 converts the adjusted quadrature modulated signal from digital form into analogue form.
The mixer 506 multiplies the adjusted quadrature modulated signal outputted by the D/A 505, by a local oscillation signal Lo that is a carrier, to generate a transmission signal. It is noted that a local oscillator that generates the local oscillation signal Lo is not shown in FIG. 14.
The power supply section 508 supplies a driving electric power to the amplification section 507.
The amplification section 507 amplifies the transmission signal by using the electric power supplied from the power supply section 508. FIG. 17 is a graph showing the relation between the quadrature modulated signal inputted to the distortion correction section 502 and the output signal of the amplification section 507. As shown in FIG. 17(a), the electric power IN of the quadrature modulated signal inputted to the distortion correction section 502 and the electric power POUT of the output signal of the amplification section 507 are corrected so as to have a linear relation therebetween by the aforementioned process of the distortion correction section 502. In addition, as shown in FIG. 17(b), a shift ΔΦ between the phase of the quadrature modulated signal inputted to the distortion correction section 502 and the phase of the output signal of the amplification section 507 is corrected so as to be constant by the aforementioned process of the distortion correction section 502. In other words, the transmission signal inputted to the amplification section 507 is adjusted based on the LUT of FIG. 16, thereby correcting a distortion of the transmission signal as a whole in the radio transmission apparatus 500 (see FIGS. 15 to 17).
The transmission signal amplified by the amplification section 507 is sent to the antenna 510 through the isolator 509.
The antenna 510 transmits the transmission signal amplified by the amplification section 507, to the exterior space.
The isolator 509 suppresses a variation of the output impedance of the amplification section 507, and blocks a reflected wave that returns from the antenna 510 thereto, such that the reflected wave does not reach the amplification section 507.
As described above, in the conventional radio transmission apparatus 500, the isolator 509 that is inserted between the amplification section 507 and the antenna 510 suppresses the variation of the output impedance of the amplification section 507 such that the reflected wave, which returns from the antenna 510, does not reach the amplification section 507. By so doing, as shown in FIG. 15, the input/output characteristic of the amplification section 507 does not vary. As a result, the conventional radio transmission apparatus 500 can maintain the desired input/output characteristic shown in FIG. 17, and hence can meet the standards concerning distortion such as ACP.
FIG. 18 is a view showing an example of a conventional radio transmission apparatus 600 that modulates data using a polar modulation scheme and transmits the modulated data. The conventional radio transmission apparatus 600 will be described below with reference to FIG. 18. The radio transmission apparatus 600 includes an amplitude/phase extraction section 601, an amplitude signal distortion correction section 602, a phase signal distortion correction section 603, a D/A 604, a D/A 605, a distortion control section 606, an LUT holding section 607, a mixer 608, an amplitude signal amplification section 609, a DC power supply section 610, an amplification section 611, an isolator 612, and an antenna 613.
The amplitude/phase extraction section 601 extracts an amplitude component of a signal for modulation to generate an amplitude signal, and extracts a phase component of the signal for modulation to generate a phase signal.
The LUT holding section 607 holds an LUT for correcting a distortion of an output signal of the amplification section 611.
Here, the input/output characteristic of the amplification section 611 will be described prior to description of the LUT of the LUT holding section 607. FIG. 19 shows the input/output characteristic of the amplification section 611. In FIG. 19(a), the horizontal axis indicates a driving voltage VIN supplied to the amplification section 611, and the vertical axis indicates a voltage VOUT of the output signal of the amplification section 611. As shown in FIG. 19(a), due to the influence of a saturation region, the relation between the driving voltage VIN (amplitude signal voltage) supplied to the amplification section 611 and the voltage VOUT of the output signal (polar modulated signal) of the amplification section 611 becomes nonlinear. In FIG. 19(b), the horizontal axis indicates the driving voltage VIN (amplitude signal voltage) supplied to the amplification section 611, and the vertical axis indicates a shift ΔΦ between the phase of the phase signal inputted to the amplification section 611 and the phase of the output signal (polar modulated signal) of the amplification section 611. As shown in FIG. 19(b), due to the influence of a saturation region, the shift ΔΦ between the phase of the phase signal inputted to the amplification section 611 and the phase of the output signal of the amplification section 611 varies and does not become constant.
FIG. 20 represents the LUT, held by the LUT holding section 607, by using two graphs for convenience of explanation. The graph of FIG. 20(a) is used for adjusting the driving voltage VIN (amplitude signal voltage) supplied to the amplification section 611. In the graph of FIG. 20(a), the horizontal axis indicates a voltage IN of an input signal of the amplitude signal distortion correction section 602, and the vertical axis indicates a gain (OUT/IN) given to the voltage IN of the input signal by the amplitude signal distortion correction section 602. The graph of FIG. 20(b) is used for adjusting the phase of the phase signal inputted to the amplification section 611. In the graph of FIG. 20(b), the horizontal axis indicates the driving voltage VIN supplied to the amplification section 611, and the vertical axis indicates a phase variation Δphase given by the phase signal distortion correction section 603 to the phase signal inputted to the phase signal distortion correction section 603.
The distortion control section 606 refers to the LUT of the LUT holding section 607 and controls the amplitude signal distortion correction section 602 and the phase signal distortion correction section 603, thereby causing: the amplitude signal distortion correction section 602 to adjust the amplitude signal generated by the amplitude/phase extraction section 601; and the phase signal distortion correction section 603 to adjust the phase signal generated by the amplitude/phase extraction section 601.
In accordance with the control of the distortion control section 606, the amplitude signal distortion correction section 602 adjusts the voltage (amplitude) of the amplitude signal generated by the amplitude/phase extraction section 601, and outputs the adjusted amplitude signal. Specifically, the amplitude signal distortion correction section 602 gives the amplitude signal a gain in accordance with its voltage (see FIG. 20(a)), to generate the adjusted amplitude signal.
In accordance with the control of the distortion control section 606, the phase signal distortion correction section 603 adjusts the phase of the phase signal generated by the amplitude/phase extraction section 601, and outputs the adjusted phase signal. Specifically, the phase signal distortion correction section 603 gives the phase signal a phase variation Δphase in accordance with the driving voltage VIN supplied to the amplification section 611 (see FIG. 20(b)), to generate the adjusted phase signal.
The D/A 604 converts the adjusted amplitude signal generated by the amplitude/phase extraction section 601, from digital form into analogue form.
The D/A 605 converts the adjusted phase signal generated by the amplitude/phase extraction section 601, from digital form into analogue form.
The mixer 608 generates a signal that is obtained by multiplying the adjusted phase signal outputted by the D/A 605, by a local oscillation signal Lo that is a carrier. It is noted that a local oscillator that generates the local oscillation signal Lo is not shown in FIG. 18.
The amplitude signal amplification section 609 amplifies the adjusted amplitude signal outputted from the D/A 604, by using the electric power of the DC power supply section 610, to generate the driving voltage VIN that is to be supplied to the amplification section 611.
The amplification section 611 amplifies the output signal (PM signal) of the mixer 608 by using the driving voltage VIN (AM signal) supplied from the amplitude signal amplification section 609, to generate the polar modulated signal.
FIG. 21 is graphs respectively showing the relation between the amplitude signal inputted to the amplitude signal distortion correction section 602 and the output signal (polar modulated signal) of the amplification section 611 and the relation between the phase of the phase signal inputted to the phase signal distortion correction section 603 and the phase of the output signal (polar modulated signal) of the amplification section 611. As shown in FIG. 21(a), the voltage IN of the amplitude signal inputted to the amplitude signal distortion correction section 602 and the voltage VOUT of the output signal of the amplification section 611 are corrected so as to have a linear relation therebetween by the aforementioned process of the amplitude signal distortion correction section 602. In addition, as shown in FIG. 21(b), the shift ΔΦ between the phase of the phase signal inputted to the phase signal distortion correction section 603 and the phase of the output signal of the amplification section 611 are corrected so as to be constant by the aforementioned process of the phase signal distortion correction section 603. In other words, the phase signal inputted to the amplification section 611 and the amplitude signal supplied as the driving voltage VIN to the amplification section 611 are adjusted by using the LUT of FIG. 20, thereby correcting a distortion of the polar modulated signal as a whole in the radio transmission apparatus 600 (see FIGS. 19 to 21).
The polar modulated signal generated by the amplification section 611 is sent to the antenna 613 through the isolator 612.
The antenna 613 transmits the polar modulated signal generated by the amplification section 611, to the exterior space.
The isolator 612 suppresses a variation of the output impedance of the amplification section 611, and blocks a reflected wave that returns from the antenna 613 thereto, such that the reflected wave does not reach the amplification section 611.
As described above, in the conventional radio transmission apparatus 600, the isolator 612 that is inserted between the amplification section 611 and the antenna 613 suppresses the variation of the output impedance of the amplification section 611 such that the reflected wave, which returns from the antenna 613, does not reach the amplification section 611. By so doing, the input/output characteristic of the amplification section 611 (see FIG. 19) does not vary. As a result, the conventional radio transmission apparatus 600 can maintain the desired input/output characteristic shown in FIG. 21, and hence can meet the standards concerning distortion such as ACP. The technology for suppressing a variation of the output impedance of an amplification section using an isolator as in the aforementioned conventional radio transmission apparatus 600 is described in paragraphs [0002] to [0005], [0016] and the like of Japanese Laid-Open Patent Publication (translation of PCT application) No. 2005-518745.
However, an isolator requires a large area to be mounted, and needs to have a certain height due to use of a magnet. In addition, the isolator causes an insertion loss and decreases the electric power efficiency of a circuit. Thus, in the above conventional radio transmission apparatuses 500 and 600, it is difficult to reduce the size and the thickness thereof because the isolator is used therein. Further, it is difficult to increase the electric power efficiency.